Hmm, tricky. Let’s assume you have some way to keep them cool, which is not going to be easy, since they will be decaying immediately and emitting fairly substantial amounts of high-energy radiation, and as soon as any protons form (from the decay) any collisions between them and neutrons will start forming atomic nuclei, with additional large releases of energy, on the fusion scale. You need some way to immediately get rid of the protons before they can have any collisions with the neutrons. And actually the neutrons themselves will, as soon as they collide, also stick together with fusion-level releases of energy. It’s hard to see how this experiment lasts long enough for any light emission or absorption, which is what you’re after.
If we could somehow turn off the strong force, so they just bounced off each other and did not decay, then I guess we could observe them. (They bounce off each other for the same reason atoms bounce off each other, which has nothing to do with any force, but is rather a consequence of the Pauli exclusion principle, which says two fermions (e.g. the electrons in two different atoms, or two neutrons) can’t be in the same place in the same state, which essentially means with similar energy.)
So what we’ve got is a cold neutron gas in a box. I’m going to guess it would look like a silvery-white translucent gas. What you have in this case is something like what a metal looks like: a collection of particles in a box. There will be a very large number of “particle in a box” translational states, very closely spaced (because this is a macroscopic box). The neutrons will fill them up to some level, the Fermi level, because they are fermions, just like the electrons in a chunk of metal fill up the conduction band to the Fermi level. The gas can absorb and re-emit (i.e. scatter or reflect) light at pretty much any wavelength, because the levels above the Fermi level are still very closely spaced, so almost any size quantum of energy can be absorbed. This is what happens in a metal, and it’s why metals have a silvery-white appearance.
Now, one complication is that neutrons have no charge, so any absorption of photons has to go through the much weaker interaction of the electromagnetic field with the neutron’s magnetic dipole moment (this is similar to the way nuclei absorb radio waves in a nuclear magnetic resonance experiment). Hence I would guess that even with all those available energy levels the intrinsic absorption cross section will be very low – hence, translucent silvery-white gas.
I guess if the temperature is low enough and we continue to magically turn off the strong force, we might get a liquid, held together by weak magnetic dipole interactions. Whether it could crystallize at zero pressure (1 atm is zero as far as atomic energy scales go) I have no idea. Like He, it might be necessary to apply a little pressure even as T->0 to get crystallization. One imagines it would then look like the ground state of the 3D Ising model, all the spins lined up.